The synthesis of hierarchical high-silica beta zeolites in NaF media

Article abstract of DOI:10.1039/c8ra09347d

An aerosol-assisted hydrothermal method has been applied to synthesizing hierarchical beta zeolites with SiO₂/Al₂O₃ ratios ranging from 44 to 392 in NaF media. Two different morphologies, including nano-aggregates with interparticle mesopores and plate-like zeolites with intracrystalline mesopores, can be formed depending on the SiO₂/Al₂O₃ ratios of the synthesis gels. A possible mechanism for the formation of the beta zeolites has been proposed. The obtained beta zeolites show hierarchical pores, good Al species distribution and less internal defect sites. Evaluation, by the cracking of 1,3,5-triisopropylbenzene (1,3,5-TIPB), is consistent with the acidic properties and the pore structure of beta zeolites with different ratios of SiO₂/Al₂O₃.

Full text of DOI:10.1039/c8ra09347d

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The synthesis of hierarchical high-silica beta
zeolites in NaF media†
Cite this: RSC Adv., 2019, 9, 3653
*
Guang Xiong,
Miaomiao Feng,
Jiaxu Liu,
Qingrun Meng, Liping Liu
and Hongchen Guo
An aerosol-assisted hydrothermal method has been applied to synthesizing hierarchical beta zeolites with
SiO2/Al2O3 ratios ranging from 44 to 392 in NaF media. Two different morphologies, including nano-
aggregates with interparticle mesopores and plate-like zeolites with intracrystalline mesopores, can be
formed depending on the SiO2/Al2O3 ratios of the synthesis gels. A possible mechanism for the
formation of the beta zeolites has been proposed. The obtained beta zeolites show hierarchical pores,
good Al species distribution and less internal defect sites. Evaluation, by the cracking of 1,3,5-
triisopropylbenzene (1,3,5-TIPB), is consistent with the acidic properties and the pore structure of beta
zeolites with different ratios of SiO2/Al2O3.
Received 13th November 2018
Accepted 15th January 2019
DOI: 10.1039/c8ra09347d
rsc.li/rsc-advances
media containing the template TEAOH and the mineralization
agent HF.10 The same method has been applied to the synthesis
of heteroatom BEA zeolites such as Sn, Ti, Mn-beta.15–19 Both the
Introduction
A beta zeolite is a large-pore material with a three-dimensional
structure of 12-membered ring channels.1 It has been used in
the petrochemical industry, and in ne chemistry and adsorp-
tion,2,3 owing to its large pore size, strong acid sites and high
chemical and thermal stability. In particular, synthesis of the
high-silica beta zeolite has drawn signicant attention owing to
its unique acid density, large pore size and high hydrophobicity.
A high-silica beta zeolite with a low acid density exhibited
a superior performance in methanol for the olen (MTO) reac-
tion, providing good product selectivity and a long catalytic
lifetime.4–6 In addition, high-silica beta zeolites exhibited
a lower deactivation rate for the methylation of phenol owing to
their suitable acidity.7 Moreover, the high-silica beta zeolite also
exhibited a high isosorbide yield in the dehydration of sorbitol
in water,8 and an excellent ability for large-sized volatile organic
compound (VOC) adsorption owing to the high
hydrophobicity.9
Beta zeolites with Si/Al ratios from 5 to 100 can be synthe-
sized through crystallization of mixtures including aluminosil-
icate sol–gel solution and tetraethylammonium hydroxide (TEA)
in alkaline media.10 However, the synthesis of high-silica beta
zeolites (Si/Al > 100) is relatively difficult. In alkaline systems,
a pure silica beta has been synthesized using 4,40-trimethyle-
nebis (TMP) and other quaternary ammonium cations.11,12 In
most cases deboronated or dealuminated beta zeolites are used
as seeds.13,14 Pure silica beta can be synthesized in near-neutral
uoride and the hydroxide anion can be used as the mineral-
izer.20,21 However, there are clear distinctions between the
zeolites produced in uoride and hydroxide synthesis
systems.22,23 In a hydroxide media, a lot of internal defect sites
are formed in the silica beta zeolite, which result in their
hydrophilicity properties.9,24 However, beta zeolites synthesized
in a uoride system are highly hydrophobic owing to the lack of
structural defects.
The use of the uorine sources such as HF and NH4F, leads
to a long crystallization time and large crystal sizes.15,25 Large-
sized beta zeolites with a microporous structure hinder the
approachability of the active sites by the large-sized reactant.
Therefore, it would be signicant to synthesize a high-silica
zeolite beta that possessed hierarchical pores to improve
diffusivity,26 and hence increase the reaction rate and catalytic
lifetime.27 Recently, Wang et al. used alkaline and uorine-
containing salts as mineralization agents to synthesize
a framework defect-free beta zeolite.28 It was found that the
addition of NaF was important to decreasing the crystal size.
Many other methods have been reported to synthesize hier-
archical beta zeolites, such as hard-templating,29 so-templat-
ing,30 and post treatment methods.31 However, it is still
desirable to synthesize a hierarchical beta zeolite using a simple
and economic method without the need for a secondary
template.
The aerosol process is an industrial method that is used to
synthesize various types of mesoporous and macroporous
materials.32 In previous reports, the aerosol-assisted method
has been applied to the synthesis of various microporous
materials including TS-1, ZSM-5 and beta zeolites.33–36 Using
State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian
University of Technology, Dalian, 116024, China. E-mail: gxiong@dlut.edu.cn; Fax:
+86-411-84986340
† Electronic supplementary information (ESI) available: Additional catalytic data.
See DOI: 10.1039/c8ra09347d
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this method the amount of template and the crystallization instrument at 300 kV. A Nicolet is10 Fourier-transform infrared
time are reduced. The zeolites synthesized using the aerosol- (FT-IR) spectrometer was used to characterize the hydroxyl
assisted method exhibit good catalytic performances. The aim vibrations between 4000–3200 cmꢁ1
. Inductively coupled
of this study is to synthesize a high-silica beta zeolite with plasma (ICP) spectroscopy was used for the elemental analysis
hierarchical pores using the aerosol-assisted hydrothermal in Optima 2000 DV. Quantachrome AUTOSORB-1 apparatus
method in NaF media. The optimal synthesis conditions were was used to measure the nitrogen sorption isotherms at 77 K.
determined and the synthetic mechanism was investigated. The Brunauer–Emmett–Teller (BET), t-plot and Barrett, Joyner,
Finally, the acid sites and pore structure of the obtained beta and Halenda (BJH) methods were used to calculate and analyse
zeolites were investigated using a probe reaction, the cracking the surface area and pore volume, respectively. To analyse the
of 1,3,5-TIPB.
acidic strength distributions and total acidity of the catalysts,
a Quantachrome CHEMBET-3000 instrument was used to
obtain the NH3-TPD proles in He ow at a rate of 10 minꢁ1 to
Experimental
Materials
ꢀ
650 C. An Agilent DD2-500 MHz spectrometer was applied to
record the 27Al and 29Si MAS NMR spectra using a spinning
speed of 14 and 13 kHz, respectively. The chemical shis were
referenced to 1% Al(NO3)3 and tetramethylsilane aqueous
solution, respectively.
The raw materials were: NaAlO2 (41.0 wt% Al2O3, Sinopharm
Chemical Reagent Co. Ltd), colloidal silica (30 wt% SiO2,
Qingdao Cheng Yu Chemical Co., Ltd), sodium uoride (NaF $
98%, Tianjin Fuchen Chemical Reagents Co. Ltd), tetraethy-
lammonium hydroxide (25 wt% TEAOH, Tianjin Guangfu Fine
Chemical Research Institute), and H-beta zeolite (SiO2/Al2O3 ¼
30, Qiwangda Chemical Technology CO. Ltd, Dalian).
Catalytic test
The 1,3,5-TIPB cracking reaction was used to investigate the
pore structure and acidity of the beta zeolites. The catalyst,
0.1 g, (20–40 mesh) was added to the stainless U-tube reactor
and activated with nitrogen at 400 ꢀC for 3 h. Then, 0.3 mL 1,3,5-
TIPB (95 wt%, Aladdin) was introduced into the reactor under
a 0.1 MPa nitrogen ow. Online GC-7890T equipped with
a thermal conductivity detector (TCD) was used to analyse the
products. The temperaturesꢀof the chromatogram detector and
the injection port were 270 C. Data analysis was compared to
the literature values.37
Synthesis
Typically, the synthesis of the beta zeolite using aerosol-assisted
hydrothermal synthesis was performed as follows: NaAlO2 and
colloidal silica were dissolved in deionized water. The nal
solution composition was SiO2 : 0–0.02Al2O3 : 0–0.02Na2-
O : 25H2O. The aerosol particles were generated via an aerosol
generator (NAI-GZJ, Shanghai Naai Instrument Co., Ltd.) at
ꢀ
230 C.
The SiO2–Al2O3–Na2O amorphous powder was dried at
110 ꢀC, and then mixed with sodium uoride and tetraethy-
lammonium hydroxide (25%, TEAOH). In some cases 5 wt%
beta seeds (SiO2/Al2O3 ¼ 30) were required. The mixture, which
had a molar ratio of 1SiO2 : 0–0.02Al2O3 : 0–0.02Na2O : 0.15–
0.2NaF : 0.15–0.2 TEAOH : 3.68–4.91H2O was loaded into
a Teon-lined autoclave, aged at 120 ꢀC for 24 h, and then
Results and discussion
Optimal synthesis conditions for the beta zeolites
The synthesis conditions of the beta zeolites with various SiO2/
Al2O3 ratios are displayed in Table 1. All samples were aged at
120 ꢀC for 1 d to generate further crystal nuclei.35 It was
observed that the beta zeolites can be obtained with a relatively
ꢀ
crystallized at 150 C for 24–96 h under autogenous pressure.
The product was collected by ltration, washing, and drying at
low amount of mineralizer/template (NaF : SiO2
¼
0.15,
TEA : SiO2 ¼ 0.15). Aer optimizing the crystallization temper-
ature and time, the samples with an initial SiO2/Al2O3 ratio
between 50–100 could be obtained without the use of seeds.
When the initial ratios of SiO2/Al2O3 were increased further the
addition of the seeds was necessary.
Table 1 shows the SiO2/Al2O3 ratios of the precursor and the
BEA samples. It should be noted that the raw material (the silica
gel 30%) contains an aluminium impurity (SiO2/Al2O3 ¼ 3654).
In the absence of the seeds, the SiO2/Al2O3 ratios of the BEA
samples (B50-B100) are close to those of the raw materials.
However, the addition of the seeds (SiO2/Al2O3 ¼ 30) lead to
a further decrease in the SiO2/Al2O3 ratios of the products. The
ꢀ
ꢀ
100 C and calcined at 540 C for 10 hours. The H-form beta
zeolite was obtained by stirring the calcined zeolite into 1 M
NH4NO3 solution at 80 ꢀC for 2 h, twice. Finally, the zeolites
ꢀ
were calcined at 540 C for 6 hours.
All samples were expressed as Bx, x indicates the SiO2/Al2O3
ratio of the initial compound. The sample BS-1/2 represented
the zeolite synthesized from Ludox. The relative crystallinity
was calculated by comparing the intensity of the peak (2q ¼
22.4) of the Bx samples with that of the B50 sample.
Characterization
Rigaku D/MAX-2400 apparatus was operated at a scanning rate effect of the SiO2/Al2O3 ratios of the seeds on the XRD patterns
of 8ꢀ minꢁ1 to obtain the X-ray diffraction (XRD) patterns of the zeolites is exhibited in Fig. S1.† It can be seen that only
between the range of 2q ¼ 5–50ꢀ with CuKa radiation. The the seeds with a SiO2/Al2O3 ratio ¼ 30 are effective for the
scanning electron microscopy (SEM) images were determined synthesis of the high-silica zeolite beta. The seeds with higher
using
a FEI Nova NanoSEM450. Transmission electron SiO2/Al2O3 ratios may dissolve too quickly under the highly
microscopy (TEM) images were obtained on a FEI Tecnai F30 alkaline conditions, and therefore are not very effective in this
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Table 1 Influences of different synthesis conditions
b
SiO2/Al2O3
NaF : SiO2 and
TEAOH : SiO2
Sample
Seed/Si/wt%
Temperaturea/ꢀC
Time/d
Precursor
Products
Relative crystallinity (%)
B50
0.15
0.15
0.15
0.15
0.15
0.2
—
—
5
5
5
150
150
130
130
130
130
2
2
4
4
4
4
47
90
139
219
3052
3052
44
85
99
154
—
100
B100
B150
B300
BS-1
BS-2
97
94
82
70
79
5
392
a
b
All samples are pre crystallized at 120 ꢀC for 1 d. Determined using ICP-AES.
synthesis system. Therefore, the Al species can be introduced
Fig. 2 shows the SEM images of the BEA samples. The
from the Al source (NaAlO2), the Si source (Ludox) or the seed. morphology of the B50 sample shows the spherical nano-
The highest SiO2/Al2O3 ratio of 392 was achieved when Ludox aggregates. Upon increasing the SiO2/Al2O3 ratio to 150, the
and the seeds were used as the starting materials.
nanocrystal aggregates more tightly to form irregular particles
The XRD patterns of the BEA samples with different SiO2/ with a larger size. However, when the SiO2/Al2O3 ratio is
Al2O3 ratios are exhibited in Fig. 1. Fig. 1a and b show diffrac- increased further, the particle sizes becomes smaller and more
tion peaks that are typical for BEA – type topology.1 This indi- uniform. The BS-2 sample shows square plate-like aggregates
cates that well-crystallized beta zeolites in a broad range of SiO2/ with a size of 100 ꢂ 500 ꢂ 500 nm, which is much smaller than
Al2O3 ratios can be obtained by optimizing the synthesis those of the pure silica beta synthesized at near neutral pH with
conditions. For the BS samples, a higher crystallinity can be HF as the mineralizer.10 This is because the crystallization
achieved if the template and mineralizer amounts are increase occurred in a dense system with a high alkalinity, which facil-
appropriately.
itates the nucleation of the crystals and reduces the crystal size.
The existence of Al species is benecial to the formation of the
crystal nucleus.11 Thus, the morphology changes from
Fig. 1 XRD patterns for: (a) the samples B50-B150; and (b) the samples
B300-BS-2.
Fig. 2 SEM images of the BEA samples.
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Fig. 4 TEM images of the BS-2 sample.
The formation of the mesopore was conrmed using the
TEM image shown in Fig. 4. As shown in Fig. S2,† aer the
crystallization at 130 ꢀC for 48 h, the nano-sized crystals
aggregate together. Therefore, the hierarchical structure should
be formed by the aggregation of small crystals, thus generating
the stacking interspace. The crystal edge shows the regular
lattice fringes of the microporous zeolite, which corresponds to
the smooth edge in the SEM image (Fig. 8).
Fig. 5a shows the 29Si MAS NMR spectra of the BEA samples.
The resonance positions of ꢁ113 and ꢁ116 ppm are associated
with the (SiO)4Si groups (Q4). There are very weak signals at
ꢁ105 ppm for the samples B50-300. It was reported that the
signal at ꢁ95 to ꢁ105 ppm could be assigned to Si(OSi)3(OAl).
However, a signal for Si(OSi)3(OH) could be observed at
a similar position (ꢁ100 to ꢁ103 ppm),39 and thus cannot be
excluded. As the SiO2/Al2O3 ratio further increases, the signal at
ꢁ105 ppm can barely be observed. This indicates that the BEA
samples with a high SiO2/Al2O3 ratio synthesized in NaF media
exhibit few defects. The Fꢁ can balance the positive electricity of
the structure directing agents and the charge of the Si–Oꢁ,
which helps to reduce the amount of framework defects.40
Fig. 3 (a) N2 sorption isotherm, and (b) BJH pore size distribution of
the BEA samples.
nanocrystal aggregates to plate-like crystals as the SiO2/Al2O3
ratio increases.
Fig. 3 shows the N2 sorption isotherms of the BEA samples.
All of the samples show a type IV isotherm, including a steep
rise in the uptake at low pressure and a hysteresis loop at high
pressure. This indicates the existence of micropores and mes-
opores, respectively.38 The hysteresis loop becomes more
obvious with the increasing SiO2/Al2O3 ratio. As shown in
Fig. 3b, the distribution of the pore sizes becomes wider as the
SiO2/Al2O3 ratio increases. Table 2 lists the textural properties of
the BEA samples. By increasing the SiO2/Al2O3 ratio, the BET
surface area gradually decreases, and the external surface area
rst increases and then decreases. This could be attributed to
changes in the morphology and aggregation. Meanwhile, the
mesopore volume rises from 0.21 to 0.28 cm3 gꢁ1 along with
a decrease in the micropore volume from 0.23 to 0.09 cm3 gꢁ1. It
can be seen that the relative crystallinity of the BEA sample
decreases with the increasing SiO2/Al2O3 ratio (Table 1). This
may explain the decrease in the micropore volumes.
Table 2 Textural properties of the BEA samples
Pore volume
Surface area (m2 gꢁ1
)
(cm3 gꢁ1
)
Sample
SBET
Smicro
Sext
Vmicro
Vmeso
B50
585
539
460
447
296
460
408
294
295
183
125
131
166
152
113
0.23
0.21
0.15
0.15
0.09
0.21
0.24
0.25
0.26
0.28
B100
B150
B300
BS-2
Fig. 5 29Si (a) and 27Al MAS NMR spectra (b) of the BEA samples.
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Fig. 6 Hydroxyl-IR spectra for the BEA samples.
Fig. 7 NH3-TPD curves for the BEA samples.
Fig. 5b shows the 27Al MAS NMR spectra of the BEA samples.
The resonance bands around 55 and 0 ppm are associated with
the tetrahedrally coordinated framework Al species and the
extra-framework Al species, respectively.41 Upon a decrease of
the Al amount, the intensity of the signal around 55 ppm
decreases gradually, indicating a decrease in the amount of the
framework Al. The resonance band at 0 ppm is associated with
the extra-framework Al species. With the decrease in the
amount of Al, the overall trend of the peak intensity decreases
gradually. However, compared to the B50 and B100 samples, the
B150, B300, BS-2 samples were synthesized with seeds at a lower
temperature to prolong the crystallization time. This may cause
the generation of the extra-framework Al species in highly
asymmetric or multinomial environment, which lead to the
broad peak at 0 ppm.
Fig. 6 shows the hydroxyl-IR spectra for the BEA samples.
The samples exhibit bands at 3610 and 3735 cmꢁ1 corre-
sponding to the bridging hydroxyl groups (Si–OH–Al) and iso-
lated Si–OH, respectively. The peak intensities decrease with the
increasing SiO2/Al2O3 ratio. For the B50-300 samples, a very
weak signal at 3500 cmꢁ1 can be observed. The band is assigned
to the hydrogen bonded silanol groups. Upon further increasing
the SiO2/Al2O3 ratio, the band disappeared. This indicates that
the BEA samples with a high SiO2/Al2O3 ratio synthesized in NaF
media exhibit few internal defects.
The XRD patterns of the BEA zeolites obtained at different
crystallization times are recorded in Fig. 8a. The crystallization
curve is presented in Fig. 8b. The crystallization curve of BS-2
shows a typical S-type curve,46 including an induction period,
the growth period and the growth ending period. The long
induction period (0–24 h, 120 ꢀC) is necessary for accumulating
amorphous materials which have the short-range ordered
structure.47 The weak peaks at 2q ¼ 7.8ꢀ and 22.4ꢀ for the BEA
topology begin to appear from 24 h. During the growth period,
the peak intensities increase rapidly, indicating that the amor-
phous phase is transformed into the framework of the BEA
zeolite. The well-crystallized BEA samples can be obtained aer
the crystallization at 130 ꢀC for 96–120 h. Aer that the relative
crystallinity drops.
The morphology change of the BS-2 sample is exhibited in
the SEM images (Fig. 9). At the beginning, the precursor (0 h)
consists of irregular spherical particles surrounded by seeds. As
the temperature rises, the precursor is aggregated and is
Fig. 7 displays the NH3-TPD results for the BEA samples. The
ꢀ
B50-BS-2 samples exhibit desorption peaks at 380–450 C and
220–250 ꢀC, which are consistent with the strong and weak acid
sites, respectively. In general, the Lewis acid site is the weak acid
site. The Brønsted acid site corresponds to the strong acid site,
which is associated with the framework aluminum atom.42,43
By increasing the aluminum amount, the amounts of the
strong and weak acid sites increase gradually. Meanwhile, the
desorption temperatures shi to the high-temperature side
slightly, indicating that the acid strength becomes stronger.
Crystallization process for the beta zeolite
There have been many efforts to study the crystallization
mechanisms, including using liquid,44 solid,25 and solid–liquid
two phase transformations.45 To further understand the crys-
tallization process of the high-silica BEA zeolite, the crystalli-
zation mechanism of the BS-2 sample was investigated using
XRD and SEM techniques.
Fig. 8 XRD patterns (a), and crystallization curve (b), for the BS-2
sample obtained at different periods of time.
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gradually dissolved (120 ꢀC, 6–24 h). Aer crystallization at
In conclusion, the high-silica BEA zeolite with mesopores
ꢀ
130 C for 6 and 48 h, small crystals are generated. This mani- can be synthesized via the aerosol-assisted route in NaF media.
fests the initial formation of the Beta zeolite during the growth The nucleation and growth of the BEA samples primarily occurs
period, as conrmed using the XRD patterns. The precursor in the liquid phase.
ꢀ
disappears rapidly aer crystallization at 130 C for 48 h. The
square plate-like crystals are formed by the aggregation of small
crystals. This explains the formation of the mesopore in the
Catalytic performance
The catalytic cracking of 1,3,5-TIPB was used as a probe reaction
to evaluate the acidic properties and pore structure of the BEA
samples with different SiO2/Al2O3 ratios. There are three
successional dealkylation reactions during the cracking of 1,3,5-
TIPB. These are: 1,3,5-TIPB: 1,3,5-triisopropylbenzene, MIPB: m-
diisopropylbenzene, and Pro: propylene:37
crystal and is in agreement with the results obtained using
TEM. By further increasing crystallization time the amount of
plate-like BEA crystals is signicantly increased and the surface
of the zeolite crystal becomes rough.
When the crystallization was completed BEA crystals with
a size of 100 ꢂ 500 ꢂ 500 nm were obtained.
TIPB / Pro + MIPB
MIPB / Pro + IPB
IPB / Pro + Ben
(1)
(2)
(3)
There are some side reactions, including disproportionation,
isomerization and condensation, that is the isomerization of
MIPB forms the p-diisopropylbenzene (PIPB).48 Step 1 and 2 can
proceed on relatively weak acid sites, but step 3 mainly requires
strong acid sites.37 The dynamic diameter of 1,3,5-TIPB (0.95
nm) is larger than the largest pore diameter of the beta zeolite
(0.76 nm). The external surface is the main location for the
cracking of 1,3,5-TIPB and the existence of the mesopore
improves the diffusion of the reactant and the product.49 The
products are closely related to the acidity and the pore
structure.50
It is generally believed that the Brønsted acid sites are the
active sites in the conversion of 1,3,5-TIPB.51 The BEA zeolites
with different SiO2/Al2O3 ratios have differing amounts of
Brønsted acid sites, thus inuencing the catalytic results.52
Table 3 shows the conversions and selectivity of the BEA
samples. The conversions of the B50 and B100 samples in 1,3,5-
TIPB cracking are quite high. The B50 sample exhibits more
Brønsted acid sites with a lower SiO2/Al2O3 ratio, while B100
sample exhibits a higher external surface area and mesopore
volume. All of these factors lead to the high conversion of 1,3,5-
TIPB by the B100 and B50 samples. Upon further increasing of
the SiO2/Al2O3 ratios, the conversion rate gradually decreases,
owing to a decrease in the amount of the framework Al species,
which generates the Brønsted acid sites.
Table 3 also shows the selectivity of the BEA samples.
Benzene is the main product, indicating that most of the 1,3,5-
TIPB reactants were cracked completely. As the SiO2/Al2O3 ratio
increases, the selectivity to benzene decreases while the
Table 3 Conversions of BEA zeolites in the cracking of 1,3,5-TIPB
Catalyst
CTIPB
SPro
SBen
SIPB
SMIPB
SPIPB
B50
97.1
98.4
91.1
85.5
54.4
48.6
49.7
51.9
52.2
53.5
45.9
45.5
41.6
44.6
38.3
2.2
2.8
4.7
2.5
6.9
0.2
0.1
0.4
0.2
1.0
3.0
1.9
1.3
0.4
0.3
B100
B150
B300
BS-2
Fig. 9 SEM images of the BS-2 sample obtained after different periods
of time.
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Conclusions
In summary, hierarchical beta zeolites with various SiO2/Al2O3
ratios have been synthesized in NaF media using the aerosol-
assisted route. The synthesis of the beta zeolites mainly fol-
lowed the liquid-phase mechanism. With the increase of the
SiO2/Al2O3 ratio, the morphology changes from spherical nano-
aggregates to plate-like crystals with a size of 100 ꢂ 500 ꢂ
500 nm. All BEA samples exhibit hierarchical-pores, which is
the key to improving the catalytic performance. The evaluation
of 1,3,5-TIPB cracking is consistent with the acid sites and pore
structure of the beta zeolites. Beta zeolites with hierarchical
pore structures are promising catalysts for the conversion and
adsorption of large organic molecules.
15 T. Blasco, M. A. Camblor, A. Corma, P. Esteve, A. Martinez,
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Conflicts of interest
There are no conicts to declare.
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This work was nancially supported by the National Science
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